Missile science: a scientist explains the mission of reference of the European Space Agency in Mercury

The European Space Agency has launched its BepiColombo mission on planet Mercury from its space port near the equator in Kourou, French Guiana, on 20 October. My involvement in the mission means that I will follow the journey with anxiety, while the spacecraft carries out a series of difficult maneuvers, culminating in its final approach to Mercury in 2025.

The mission arrives 25 years after a group of scientists proposed to the ESA to send a probe to Mercury and 18 years after ESA had approved the project as a "pivotal" mission. This is the category of world-class, scientifically excellent missions that require significant development of new technologies. The previous missions of the cornerstone of the ESA include the mission of the Rosetta comet and the LISA Pathfinder gravitational wave observatory.

But why Mercury? It is a disconcerting planet. The NASA MESSENGER orbit (2011-2015) has revealed many reasons why scientists are eager to learn more. These include the abnormally large core of the planet – we do not know why it is still melted and able to generate a magnetic field, unlike that of Mars or Venus. Another mystery is the abundance of volatile (largely unidentified) substances on its surface. These should not have been incorporated into a planet that has formed so close to the sun as Mercury is.

Missile science

The initial course of BepiColombo after three days of orbiting the Earth for checkouts will be an elliptical orbit on the sun. This will start by bringing it into the Earth's orbit. But at the beginning of 2019, it will cross it for most of the year. He then returned to the interior before approaching the Earth very much in April 2020.

BepiColombo launch and separation timeline. Photo credit: ESA

At that time he will do a "gravity-assist" flight – using Earth's gravity to swing inward toward Venus. There will also be a gravitational flight over Venus when it arrives in 2020, followed by another in 2021 to send it to Mercury. Thus, in 2021-2025 there will be a series of six similar Mercury flights, necessary to ensure that the spacecraft will eventually close on its target at a speed low enough to be caught in orbit around it in December 2025.

Each flyby, shown in the animation above, must be executed perfectly. Things could go wrong, especially during the launch, but I have full confidence in the capabilities of the ESA flight control team in Darmstadt, Germany.

Spacecraft stacked

The mission, which is named after the memory of Giuseppe (Bepi) Colombo, who first proposed the gravity-assist flybys for space vehicles, is a joint venture between ESA and its Japanese counterpart, JAXA.

The spacecraft carries two orbits. The ESA is a two-meter-long unit, which amasses more than a ton, called the Mercury Planetary Orbiter, MPO. I suspect that after starting to orbit Mercury, it will inherit the name of BepiColombo or perhaps only Bepi. The Japanese orbiter is smaller and its mass is about a quarter of the ESA orbit. Originally called Mercury Magnetospheric Orbiter, MMO, in June it was awarded the name Mio, which in Japanese has connotations of safe navigation. During the cruise to Mercury, Mio will be housed inside a parasol and connected to one side of the European satellite.

The impression of the artist of BepiColombo during his flight over the Earth in April 2020. Mio can be seen hidden in his parasol. Photo credit: ESA / ATG medialab

On the other side of the orbiter is the Mercury Transfer Module, MTM. This is managed by the ESA and provides the propulsion to bring the stacked spaceship up to its orbit of Mercury. It has a 7.5-meter solar panel, whose task is to transform sunlight into electricity to power its "ionic motor". This is a propulsion device that creates thrust by accelerating the xenon gas that has been positively charged (stripping its electron atoms). This technique can provide much more boost to fuel mass than conventional chemical rockets.

The tremendous gravity of the sun means that more energy is needed to enter a stable orbit on Mercury than would be needed to send the same spacecraft to Pluto much further away. For this reason, the ion disc will be operated at intervals equal to about half the duration of the cruise, mainly to slow the spacecraft down.

Unfortunately, the stacked configuration of the combined spacecraft prevents its ability to do science during planetary flights. Some scientific data will be collected, but the best images we will get during the flight will be the selfie-cams mounted on the MTM.

Coming to Mercury

Upon arrival at Mercury, at the end of December 2025, the transfer form will be disconnected. Mio, rotating at 15 rpm for stability, will then be released in a highly elliptical orbit on Mercury. As soon as this happens, JAXA will take control of Mio's operations and will guide it in its tasks by studying the magnetic field of the planet and the associated space environment.

The orbit of ESA will then discharge the parasol, its last impediment, and use its chemical thrusters to obtain a closer or more circular orbit on Mercury. From there he will study the surface of the planet using an assortment of cameras and other instruments. This should define the composition and geological history in much more detail than the smaller and less complex MESSENGER. The orbiter will also carry a magnetometer so that both he and Mio will be able to bring back the magnetic conditions in two places at the same time: an important first for a deep space mission that should teach us the speed with which the disturbances travel through the magnetic field of the planet.

It is exciting to think that BebiColombo can transform our knowledge of Mercury in a few years. And while you wait, from October 23, you will be able to listen to beautiful and suggestive music that the planet has inspired as part of the Planets 2018 project. This was established to commemorate the centenary of Gustav Holst's Suite of Planets with music inspired by science of the planets.

David Rothery, professor of planetary geosciences, The Open University.